This invention relates to the nanostructured ceramic platform for design, methods of manufacture, operation modes, and uses of micromachined devices and device arrays in general, and sensors and sensor array devices in particular.
The ability to monitor the chemical composition and physical parameters of gaseous environments, including indoor air quality, atmospheric measurements for weather forecasting, worker safety in hazardous environments and closed spaces, processing gases, and vehicle and plant exhaust, has been an important goal for several reasons. For example, the detrimental environmental effects of toxic species such as formaldehyde, carbon monoxide, ozone, hydrocarbons, chlorocarbons, nitrogen oxides and aromatics have led to the need to develop efficient, sensitive, and affordable ways of detecting the composition and presence of such toxic substances. Additionally, the efficiency of chemical processes, in terms of energy and raw material used per unit product or service delivered, relies on the ability to reliably sense deviations from the optimal processing conditions. Development of measurement and control technologies, including advanced sensor technology, is a critical component for lowering raw materials and energy consumption, improving the productivity and reducing generation of waste and pollutants. On the other hand, a primary concern for health care facilities as well as for other work place environments, are volatile organic compounds, such as alcohols, chlorinated hydrocarbons, aromatic solvents, and formaldehyde. Furthermore, humidity, temperature, and pressure are important environmental factors, which are of importance to both human comfort and to many industries and technologies, such as the production of electronic devices, precision instruments, food products, agriculture, horticulture, green houses and meteorology. Low-cost, light-weight sensors for routine balloon-borne measurements of water vapor concentration are required to provide one percent accuracy with high time resolution measurements at altitudes up to 20 km, low vapor concentration, and at temperatures down to at least minus 60xc2x0 C.
Although physical, chemical, and biochemical sensing have been extensively researched, real-time monitoring and control leaves much to be desired. The following sensor performance parameters are in need of improvement for many applications: reliability and robustness, miniaturability, real time response, sensitivity and selectivity, built-in self-calibration, survivability in harsh application environments, low cost and manufacturability. Existing sensors have several inherent limitations that limit their performance, the most restraining are: cost, lack of selectivity, low reliability and durability, slow response, false response, the need for significant power, resistance to poisoning by sulfur-containing compounds and instability (hysteresis).
Existing approaches for high temperature microsensors are based mainly on planar technologies: the functional components (sensitive element, electrodes, temperature sensor, heating structures, and passivation) are made using standard thin or thick film deposition. Although significant breakthroughs were achieved in the development and fabrication of thin film microsensors, their performance parameters, especially sensitivity, reliability and working temperature range, still limits their usability. Moreover, portable sensor arrays capable of detecting multiple toxic air pollutants, such as volatile organics, carbon monoxide, formaldehyde, nitrogen oxides, and other air components, such as carbon dioxide and humidity are not available.
Recent advances in nanotechnology can greatly improve the sensor performance due to the possibility of tailoring the microstructure and chemistry at the nanoscale level, thus enhancing the gas-solid interaction. Although significant progress was achieved in different types of microsensors, synthesis of nanostructured high surface area sensing elements on intrinsically flat non-porous micromachined substrates is still a problem. Thin film microsensors fabricated on diaphragms, which are thermally decoupled from the substrate, demonstrated significant advantages in response time, and potential for array integration in comparison with larger (thick film) sensors. Nevertheless, there are several outstanding problems with existing thin film sensors. Small amounts and low surface area of sensitive material leads to lower signal-to-noise ratio (sensitivity), and easy loss of sensitivity on exposure to sulfur-containing compounds, as well as poor overall durability.
Nanostructured materials, with their small grain size, large number of grain boundaries and high specific area present new opportunities for the development and the commercialization of the next generation of gas sensors for air quality monitoring and control with significantly improved properties. The main challenge in realizing such opportunities is tailoring the sensing materials morphology and the composition at the nanometer scale. Furthermore, it is important that the nanostructured sensing element is manufactured in a form that is amenable to sensing device architectures and with ppb reproducibility.
If these issues were resolved, significant performance advantages could be realized. Low thermal mass and fast response microsensors could allow real time process monitoring and control, as well as will enable implementation of programmable temperature/voltage modulation for higher sensitivity and stability, elimination of drift and noise, and self-calibration. This invention addresses this opportunity.
Prior publications related to producing gas sensors include those of U.S. Pat. No. 4,631,952 which teaches a method of preparing a sensor by the formation of a dispersion of conducting particles with a material capable of swelling in the presence of the liquid, gas or vapor to be sensed. U.S. Pat. No. 4,984,446 teaches the preparation of a gas detecting device by a layer by layer build up process, and U.S. Pat. No. 4,988,539 teaches an evaporation-deposition method and process for manufacturing a gas detection sensor. U.S. Pat. No. 5,387,462 teaches a method of making a sensor for gas, vapor, and liquid from a composite article with an electrically conductive surface having an array of whisker-like microstructures with an aspect ratio of 3:1 to 100:1. U.S. Pat. No. 5,345,213 teaches a method for fabrication of temperature-controlled micromachined arrays for chemical sensor fabrication and operation. Furthermore, U.S. Pat. No. 6,079,873 describes a microhotplate-based differential calorimeter for detecting gases and chemical reactions. Although these prior methods provide improved methods for producing sensors, there is still a need to develop sensors that are selective, sensitive to trace species, fast, small, accurate, reproducible, stable in extreme environments, durable, and finally affordable.
Prior publications relevant to nanoporous anodic alumina include U.S. Pat. Nos. 4,472,533; 5,174,883; 5,198,112; 5,202,290 and 5,581,091, which describe methods of depositing various materials inside the nanoscale pores of anodic alumina. Depending on the material deposited in the pores and the pore size, these materials were described as suitable for use as catalysts, microelectrodes, reverse osmosis membranes, or nanoelectric devices. However, none of these references describe the manufacture of nanotubes of sensing materials, nor do they suggest that gas sensing devices can be constructed by these methods.
This invention proposes to use intrinsic morphology of self-organized nanoporous anodic aluminum oxide (alumina), for both micromachining of the sensor substrate and for making advanced nanostructured sensing elements. The nanoporous morphology of such sensing element would enable the desired grain size, high specific area and therefore high sensitivity, while the micromachined ceramic substrate would define the sensor and the array, provides the interface with the readout and control circuit, low power consumption, enables long lifetime, capability to regeneration and manufacturability.
In one aspect, the present invention involves a sensor or sensor array platform comprising a micromachined nanostructured anodic alumina substrate, bulk nanostructured sensing element, thin film or bulk microheater, temperature detector and gas permeable sensing electrodes. In other embodiments the microheater may be the temperature detector and one of the sensing electrodes. The sensor may include other layers, for example, insulating layers. The sensor may be partially or completely coated, for example to protect the electrodes from environmental damage. The interaction between the sensor and the analyte may be physical, chemical, electronic, electrical, magnetic, structural, thermal, optical, or their combination.
The invention also includes a method of producing nanoporous sensor substrates from anodic alumina, comprising the steps of anodizing aluminum metal, or aluminum films on a substrate. These substrates include metals, silicon wafers, glass, and polymers. Anodic alumina films may be micromachined to delineate the sensor or sensor array substrate. The chemistry of the nanoporous substrate may be modified by liquid- or gas-phase processes to yield desired properties. The substrate may be annealed to create desired surface chemistry and specific surface area.
The invention also comprises a method of producing a nanostructured sensing element using deposition of materials inside the pores of anodic alumina. The method includes providing an alumina film having a plurality of elongated parallel pores, and depositing a sensing material in the pores, wherein the pores have an average diameter of 500 nm or less. The resulting nanostructured sensing element comprises one or more materials selected from the group consisting of metals, non-metals, oxides, salts, polymers, and other organic and inorganic compounds. The sensing material may be deposited in the pores, for instance, by electrodeposition, sol-gel processes, solution impregnation, spin-coating, spray-coating, gas phase physical deposition and chemical vapor deposition.
In another aspect, the invention comprises an array of nanotubes or nanowires having an average diameter of less than 500 nm, wherein the array exhibits a high sensitivity and selectivity for a physical property or analyte.
The invention also comprises methods of making sensing devices incorporating the arrays of nanowires or nanotubes of sensing materials, by providing conductive electrodes in electrical contact with at least a portion of the nanotubes or nanowires. The electrodes may comprise conductive material selected from the group consisting of gold, silver, platinum, palladium, aluminum, copper, nickel, and alloys thereof. The electrodes can be formed on the anodic alumina substrate by a wide variety of techniques, including vacuum thermal evaporation, DC and RF plasma sputtering, chemical vapor deposition, electrochemical deposition, electroless deposition, and screen printing.